Measurement of the ridge in pp and p+Pb collisions with the ATLAS detector at the LHC

Recent measurements from the ATLAS experiment of the azimuthal anisotropy of charged hadron production in $pp$ collisions at $\sqrt{s}$ = 2.76, 5.02 and 13 TeV and in $p$+Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02~TeV at the Large Hadron Collider (LHC) are presented. The measurements of the two-particle correlations (in $\Delta\eta, \Delta\phi$) using a template fitting procedure to remove correlations arising from hard-scattering processes are shown. The results on the per-trigger-particle yields, $Y$, as well as on $v_\mathrm{n}$ Fourier coefficients are presented as a function of the charged particle multiplicity, $N^\mathrm{rec}_\mathrm{ch}$, and transverse momentum, $p_\mathrm{T}$, for $n$ = 2--4. Interestingly, $v_\mathrm{2}$ values in $pp$ collisions are found to be approximately constant as a function of multiplicity and the $p_\mathrm{T}$-dependence of $v_\mathrm{n}$ is similar to that measured in Pb+Pb collisions suggesting that in small systems features characteristic of the collective anisotropic flow are also present.